Gas turbine engines generally include a gas generator comprising: a compressor section with one or more compressors for compressing air flowing through the engine; a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream; and a turbine section which includes one or more rotors for driving the compressor(s). Many engines further include an additional turbine section, known as a power turbine, located aft of the gas generator which extracts energy from the gas flow out of the gas generator to drive an external device such as a fan or a propeller.
Each of the turbines and compressors include components which are generally cylindrical or conical in shape. For example, the annular flowpath through both the compressor and the turbine is bounded by radially inner and outer casings. These casings may be static or rotating, and may or may not support blade rows.
Such components are subject to vibration during engine operation. More particularly, each component is subject to nodal diameter vibration. Nodal diameter vibration is characterized by two or more points (nodes) on the circumference of the annular structure remaining fixed while points between the nodes undulate back and forth.
Annular components tend to vibrate at a natural or resonant frequency. When these components are subjected to an excitation source vibrating at the resonant freqeuncy of the component, the component may fail. Although such failures rarely occur, a more typical problem is the secondary structural damage which may result from rubs between rotating and stationary parts. High performance gas turbine engines require close tolerances be maintained between, for example, rotating blades and the surrounding casing. If the casing vibrates, an interference or rub between the surfaces may occur. Such rubs create gaps which deteriorate engine performance.
The frequency of nodal vibration may be raised to avoid resonance by stiffening the annular components. Such stiffening may be achieved by increasing the thickness of the components. However, this can result in excessive weight and resulting performance penalties. Alternatively, a relatively narrow axial portion can be thickened by a flange-like arrangement. For example, where two annular members are joined, the connecting flange will function as a stiffening member.
In many gas turbine engine designs, there is limited space between the outer casing and surrounding nacelle or outer structure. In these engines, flanges reduce the radial space available for other components to pass therethrough.